Concurrent Programming with Clojure - INNOQ · Stefan Tilkov | innoQ Concurrent Programming with...

Stefan Tilkov | innoQ

Concurrent Programming with Clojure

Functional Programming meets the JVM

1Thursday, May 20, 2010

Stefan Tilkov

[email protected]://www.innoq.com/blog/st/

@stilkov


http://rest-http.info


SoftwareArchitekTOURMichael Stal - Christian Weyer -

Markus Völter - Stefan Tilkov

http://heise.de/developer/podcast


http://www.innoq.com


27.04.10 14:17http://upload.wikimedia.org/wikipedia/en/1/1a/Clojure-glyph.svg

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Clojure 27.04.10 14:17http://upload.wikimedia.org/wikipedia/en/1/1a/Clojure-glyph.svg

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A practical Lisp variant for the JVM Functional programming

Dynamic Typing Full-featured macro system

Concurrent programming supportBi-directional Java interop

Immutable persistent data structures


Lisp??


Lots of irritating silly parentheses?


http://lemonodor.com/archives/2007/10/youre_doing_it_wrong.html


http://xkcd.com/297/


http://www.flickr.com/photos/nicolasrolland/3063007013/


http://www.tbray.org/ongoing/When/200x/2008/09/25/-big/R0010774.jpg.html

Rich Hickey


Intro


Clojure Environment

Clojuresque (Gradle)

Leiningen


Data structuresNumbers 2 3 4 0.234

3/5 -2398989892820093093090292321

Strings "Hello" "World"

Characters \a \b \c

Keywords :first :last

Symbols a b c

Regexps #"Ch.*se"

Lists (a b c)((:first :last "Str" 3) (a b)

Vectors [2 4 6 9 23][2 4 6 [8 9] [10 11] 9 23]

Maps {:de "Deutschland", :fr "France"}

Sets #{"Bread" "Cheese" "Wine"}


Syntax


“You’ve just seen it” – Rich Hickey


Syntax(def my-set #{:a :b :c :c :c}) ;; #{:a :b :c}(def v [2 4 6 9 23])(v 0) ;; 2(v 2) ;; 6

(def people {:pg "Phillip", :st "Stefan"})(people :st) ;; "Stefan"(:xyz people) ;; nil

(+ 2 2) ;; 4(class (/ 4 3)) ;; clojure.lang.Ratio(* (/ 4 3) 3) ;; 3

(format "Hello, %s # %d" "world" 1)


Syntax

; (a 2 3)(quote (a 2 3)) ;; (a 2 3)'(a 2 3) ;; (a 2 3)

; Evaluation (eval '(format "Hello, %s" "World"))(eval (read-string "(+ 2 2)"))

(format "Hello, %s # %d" "world" 1); "Hello, World # 1"

(apply str ["Hello, %s # %d" "world" 1])


Functions(fn [x] (format "The value is %s\n" x));; user$eval__1706$fn__1707@390b755d

((fn [x] (format "The value is %s\n" x)) "Hello");; "The value is Hello"

(def testfn (fn [x] (format "The value is %s\n" x))) (testfn "Hello")

(defn testfn [x] (format "The value is %s\n" x))(testfn "Hello")


Functions

(defn even [x] (= 0 (rem x 2))) (even 4) ;; true (def even-alias even)(even-alias 2) ;; true

(defn every-even? [l] (every? even l))(every-even? '(2 4 6 8 9)) ;; false(every? #(= 0 (rem % 2)) '(2 4 6 8 9)) ;; false


Functions(defn make-counter [initial-value] (let [current-value (atom initial-value)] (fn [] (swap! current-value inc))))

(def counter1 (make-counter 0))(counter1) ;; 1(counter1) ;; 2

(def counter2 (make-counter 17))(counter1) ;; 3(counter2) ;; 18(counter1) ;; 4(counter2) ;; 19


Recursion

(defn reduce-1 [f val coll] (if (empty? coll) val (reduce-1 f (f val (first coll)) (rest coll))))

(reduce-1 + 0 [1 2 3 4]) ;; 10(reduce-1 + 0 (range 5)) ;; 10(reduce-1 + 0 (range 50)) ;; 1225(reduce-1 + 0 (range 50000)) ;; java.lang.StackOverflowError


(defn reduce-2 [f val coll] (if (empty? coll) val (recur f (f val (first coll)) (rest coll))))

(defn reduce-1 [f val coll] (if (empty? coll) val (reduce-1 f (f val (first coll)) (rest coll))))

Recursion

(reduce-2 + 0 [1 2 3 4]) ;; 10(reduce-2 + 0 (range 5)) ;; 10(reduce-2 + 0 (range 50)) ;; 1225(reduce-2 + 0 (range 50000)) ;; 1249975000


Example(ns sample.grep "A simple complete Clojure program." (:use [clojure.contrib.io :only [read-lines]]) (:gen-class))

(defn numbered-lines [lines] (map vector (iterate inc 0) lines))

(defn grep-in-file [pattern file] {file (filter #(re-find pattern (second %)) (numbered-lines (read-lines file)))})

(defn grep-in-files [pattern files] (apply merge (map #(grep-in-file pattern %) files)))

(defn print-matches [matches] (doseq [[fname submatches] matches, [line-no, match] submatches] (println (str fname ":" line-no ":" match)))) (defn -main [pattern & files] (if (or (nil? pattern) (empty? files)) (println "Usage: grep <pattern> <file...>") (do (println (format "grep started with pattern %s and file(s) %s" pattern (apply str (interpose ", " files)))) (print-matches (grep-in-files (re-pattern pattern) files)) (println "Done."))))


Macros

(log "Hello, World") Tue Apr 27 19:06:43 CEST 2010: Hello, World

(def *debug* true)

(defn log [msg] (if *debug* (printf "%s: %s\n" (java.util.Date.) msg)))

(log (format "Hello, World %d" (* 9 9))))Tue Apr 27 19:06:45 CEST 2010: Hello, World 81


Macros

(defmacro log [body] (if *debug* `(printf "%s: %s\n" (java.util.Date.) ~body)))

(log "Hello, World") Tue Apr 27 19:06:43 CEST 2010: Hello, World

(macroexpand '(log "Hello, World"))

(def *debug* true)

(if user/*debug* (printf "%s: %s\n" (java.util.Date.) "Hello, World"))

(macroexpand '(log (format "Hello, World %d" (* 9 9))))(if *debug* (printf "%s: %s\n" (java.util.Date.) (format "Hello, World %d" (* 9 9))))


Macros

(defmacro with-debug [body] `(binding [*debug* true] ~body))

Tue Apr 27 19:22:35 CEST 2010: Hello, WorldTue Apr 27 19:22:35 CEST 2010: Clojure rocks

(with-debug (log "Hello, World") (log "Clojure rocks"))

(binding [*debug* false] (log "Hello, World"))


Macros(defmacro with-debug [body] `(binding [*debug* true] ~body))

(macroexpand '(binding [*debug* true] (log "Hello, World")))

(let*[] (clojure.core/push-thread-bindings (clojure.core/hash-map (var *debug*) true))(try (log "Hello, World") (finally (clojure.core/pop-thread-bindings))))


Lots of other cool stuff

Persistent data structuresSequences

Support for concurrent programmingDestructuring

List comprehensionsMetadata

Optiional type informationMultimethods

Pre & Post ConditionsProtocols (1.2)

Extensive core and contrib libraries…


State


4711: Personfirst: Johnlast: Smith

0815: Personfirst: Janelast: Doe

The Problem!


Immutabilityuser> (def v (apply vector (range 10)))

user> (assoc v 1 99)[0 1 2 3 4 5 6 7 8 9]user> v#'user/v

[0 99 2 3 4 5 6 7 8 9]

[0 1 2 3 4 5 6 7 8 9]user> v

user> (def v2 (assoc v 1 99))

user> v2#'user/v2

[0 99 2 3 4 5 6 7 8 9]


user> (def v (apply vector (range 10)))user> (def v2 (assoc v 1 99))

9

4

7

0 1 6 83 5

2

v

99

v2


Persistent Data Structures

Pure functional programming modelEfficient implementation

Structural sharingThread-safe

Iteration-safeBased on Bit-partioned hash tries

“Transient” data structures if needed


Performance Guarantees

hash-map sorted-map hash-set sorted-set vector queue list lazy seq

conj near-constant

logarithmic

near-constant

logarithmic

constant (tail)

constant (tail)

constant (head)

constant (head)

assoc near-constant

logarithmic

- - near-constant

- - -

dissoc near-constant

logarithmic

- - - - - -

disj - - near-constant

logarithmic

- - - -

nth - - - - near-constant

linear linear linear

get near-constant

logarithmic

near-constant

logarithmic

near-constant

- - -

pop - - - - constant (tail)

constant (head)

constant (head)

constant (head)

peek - - - - constant (tail)

constant (head)

constant (head)

constant (head)

count constant constant constant constant constant constant constant linear


Sequences

Standard API for everything sequencable

CollectionsStringsNative Java arraysjava.lang.IterableAnything that supportsfirst, rest, cons


Sequences

Standard API for everything sequencable“Lazy” sequences

(def n (iterate (fn [x] (+ x 1)) 0))(def fives (map #(* 5 %) n))(take 10 fives)


Sequences

Standard API for everything sequencable“Lazy” sequencesExtensive library

applybutlastconcatconscycledistinctdoalldorundoseqdropdrop-lastdrop-whileempty?every?ffirstfile-seqfilterfirstfnextfor

interleaveinterposeintointo-arrayiterateiterator-seqkeyslastlazy-catlazy-seqline-seqmapmapcatnextnfirstnnextnot-any?not-emptynot-every?nth

nthnextpartitionpmaprangere-seqreduceremoverepeatrepeatedlyreplacereplicaterestresultset-seqreverserseqrsubseqsecondseqseq?seque

setsomesortsort-bysplit-atsplit-withsubseqtaketake-nthtake-whileto-array-2dtree-seqvalsvecwhen-firstxml-seqzipmap…


Concurrency Support


Core Ideas

Everything immutableShared state for reading

No changes to shared stateIsolated threads

Re-use of platform facilitiesJava Integration

(java.util.concurrent.Callable)


def & binding

(def some-var 10)(binding [some-var 30] (println some-var)) ;; 30

(def some-var 10)(println some-var) ;; 10

(binding [some-var some-var] (println some-var) ;; 10 (set! some-var 30) (println some-var)) ;; 30


Atoms

(def a (atom "Initial Value")) (println @a) ;; "Initial Value"

(swap! a #(apply str (reverse %)))(println @a) ;; "eulaV laitinI"

(swap! a #(apply str (reverse %)))(println @a) ;; "Initial Value"


Atoms(defn run-thread-fn [f] (.start (new Thread f)))

(defn add-list-item [coll-atom x] (swap! coll-atom #(conj % x)))

(def int-list (atom ())) ;; ()(run-thread-fn #(add-list-item int-list 5)) ;; (5)(run-thread-fn #(add-list-item int-list 3)) ;; (3 5)(run-thread-fn #(add-list-item int-list 1)) ;; (1 3 5)

(def int-list (atom ())) ;; ()(let [run-fn (fn [x] (run-thread-fn #(add-list-item int-list x)))] (doall (map run-fn (range 100)))) ;; (98 97 96 ... 0)


Refs

(defn make-account [balance owner] {:balance balance, :owner owner})

(defn withdraw [account amount] (assoc account :balance (- (account :balance) amount)))

(defn deposit [account amount] (assoc account :balance (+ (account :balance) amount)))


Refs

(defn init-accounts [] (def acc1 (ref (make-account 1000 "alice"))) (def acc2 (ref (make-account 1000 "bob"))) (def acc3 (ref (make-account 1000 "charles"))))

(defn transfer [from to amount] (dosync (alter from withdraw amount) (alter to deposit amount)))


Refs(init-accounts)

(do (run-thread-fn #(transfer acc1 acc2 100)) (transfer acc3 acc1 400))

acc1: {:balance 1000, :owner "alice"}acc2: {:balance 1000, :owner "bob"}acc3: {:balance 1000, :owner "charles"}



Refs

(defn slow-transfer [from to amount] (dosync (sleep 1000) (alter from withdraw amount) (alter to deposit amount)))

(do (run-thread-fn #(slow-transfer acc1 acc2 100)) (transfer acc3 acc1 400))




Software Transactional Memory (STM)

Multi-version concurrency control (MVCC)Atomic changes to multiple refs

Non-blocking, retry-based“Read committed”

Can't help with non-pure functionsWorks with atoms and agents

ref-set altercommuteensurederef/@ throw


Software Transactional Memory

ref-set

alter

commute

ensurederef/@ Reads value of reference, blocks none

Reads value of reference, blocks writers

Reads value of reference, blocks none,delayed write, last writer wins

Changes reference to new value, blocks writers

Atomically reads, computes, sets reference value, blocks writers

throw Rolls back transaction


AgentsAsynchronous execution

Run on java.util.concurrent thread pool

await await-forsend-offsendagent

(let [my-agent (agent 0) slow-fn (fn [x] (sleep 1000) (inc x))] (send my-agent slow-fn) (println @my-agent) (sleep 2000) (println @my-agent));; 0;; 1

deref/@


Agents

await

await-for

send-off

send

agent

deref/@

Creates agent with initial value

Dispatch function to agent for execution

Dispatch long-running function

Read agent value

Wait for agent to execute function(s) dispatched from current thread

Same as await, but with timeout


Validators(def some-var 10)

(set-validator! #'some-var #(< % 100))

(def some-var 101) ;; Invalid reference state ;; [Thrown class java.lang.IllegalStateException]

(def some-var)(defn limit-validator [limit] (fn [new-value] (if (< new-value limit) true (throw (Exception. (format "Value %d is larger than limit %d" new-value limit))))))

(set-validator! #'some-var (limit-validator 100))(def some-var 101);; Value 101 is larger than limit 100;; [Thrown class java.lang.Exception]


Watchers(def *a* (atom 0))(def *events* (atom ()))

(defn log-event [coll s] (swap! coll conj s))

(log-event *events* "some event") ;; ("some event")(log-event *events* "yet another event") ;; ("yet another event" "some event")

(defn log-value-change [key ref old new] (if (= key :log) (log-event *events* (format "value of %s changed from %d to %d" ref old new))))

(log-value-change :log 'x 0 1);; ("value of x changed from 0 to 1" "yet another event" "some event")(add-watch a :log log-value-change)(swap! a inc) ;; 1

(deref *events*) ;; ("value of clojure.lang.Atom@59829c6b changed from 0 to 1";; "value of x changed from 0 to 1" "yet another event" "some event")


Futures & Promisesuser> (doc future)-------------------------clojure.core/future([& body])Macro Takes a body of expressions and yields a future object that will invoke the body in another thread, and will cache the result and return it on all subsequent calls to deref/@. If the computation has not yet finished, calls to deref/@ will block.

user> (doc promise)-------------------------clojure.core/promise([]) Alpha - subject to change. Returns a promise object that can be read with deref/@, and set, once only, with deliver. Calls to deref/@ prior to delivery will block. All subsequent derefs will return the same delivered value without blocking.


thread-localglobal shared

def bindingset!

single multiple

sync async

atom

dosyncref

agent


Summary

Built on immutablity from the ground upPowerful collections

Extensive sequence libraryBuilt-in concurrency primitives


01 2010start date 0905

01 2010start date 05

01 201009


Java Integration


Clojure → Java(new java.lang.String "Hello")(java.lang.String. "Even quicker")(java.io.File/separator)(import '(java.io InputStream File)) (File/separator)(. System/out println "Hello")(.println System/out "Hello")

(every? #(instance? java.util.Collection %) '([1 2] '(1 2) #{1 2}));; true

(defn blank? [s] (every? #(Character/isWhitespace %) s))(blank? "some string") ;; false(blank? "") ;; true


Clojure ↔ Java

(Collections/sort java-collection (make-comparator #(. %1 compareTo %2)))

;; #<Vector [Alpha, Beta, Gamma]>

(defn make-comparator [compare-fn] (proxy [java.util.Comparator] [] (compare [left right] (compare-fn left right))))

(import '(java.util Vector Collections))

(def java-collection (Vector.))(doto java-collection (.add "Gamma") (.add "Beta") (.add "Alpha"));; #<Vector [Gamma, Beta, Alpha]>


Clojure ← Javapackage com.innoq.test;public interface ClojureInterface { String reverse(String s);}

package com.innoq.test;public class ClojureMain { public static void main(String[] args) { ClojureInterface cl = new ClojureClass(); System.out.println("String from Clojure: " + cl.reverse("Hello, World")); }}

(ns com.innoq.test)

(gen-class :name com.innoq.test.ClojureClass :implements [com.innoq.test.ClojureInterface] :prefix class-)

(defn class-reverse [this s] (apply str (reverse s)))


http://clojure.org/

http://peepcode.com/products/functional-programming-with-clojure

http://vimeo.com/channels/fulldisclojure

#clojure freenode

[email protected]

build.clojure.org

http://en.wikibooks.org/wiki/Clojure

http://technomancy.us/136

Books

http://stuartsierra.com/

http://www.bestinclass.dk/index.php/blog/

http://technomancy.us/

http://kotka.de/blog/

http://blog.fogus.me/

BlogsCore

Screencasts

http://www.assembla.com/wiki/show/clojure/Getting_Started

http://github.com/relevance/labrepl


http://build.clojure.org/

http://build.clojure.org/

Q&A

Stefan [email protected]

http://www.innoq.com/blog/st/Twitter: stilkov

http://xkcd.com/224/


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